[Moved] Instrumentation amplifier

[PRIYADHARSHINI PALANISAMY]

Hi i want to design the instrumentation amplifier with varying gain ..
my input voltage is 0.5v
i m using lm324
when i vary my Rgain as 1.3k it produces the output 8.20v
but according to the formula it should produce 5.00v
what is the reason for this?
here i have upload my schematic

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After that i have replace this lm324 with MCP604..When R gain reaches to zero it produces 4.2v at the output...but theoretically it s not correct....what is the mistake have i done here?

 

[esp1]

Re: Instrumentation amplifier

hi,
I would say the internal 1K resistor values are too low, go for 10K or 47K.
The 5K pot is much to coarse, use a ~1K.

Modify the bridge so that the common mode voltage is equal, say 4.50V.

E

 

[crutschow]

As ESP1 noted, your resistors are too low in value. Most op amps like an output load no lower than 2kΩ total. Try at least 10kΩ.

The gain of that circuit is (1 + 2R2 / R6) (for R2 = R11). Don't know how you got your gain values. :?:

 

[PRIYADHARSHINI PALANISAMY]

I have calculated the gain as follows
vout/v2-v1=( 1+ 2R2 / R6) R1/R5 (i.e) [R1=R9=R5=R10]

 

[crutschow]

Check your power supply polarities. I think the connections to amp 2 and 3 are incorrect.

 

[PRIYADHARSHINI PALANISAMY]

Re: Instrumentation amplifier

hi,
I would say the internal 1K resistor values are too low, go for 10K or 47K.
The 5K pot is much to coarse, use a ~1K.

Modify the bridge so that the common mode voltage is equal, say 4.50V.

E

I have to make that the differential amplifier input at the both nodes 4.50v.right?

 

[esp1]

I have to make that the differential amplifier input at the both nodes 4.50v.right?

hi,
Its important that the steady state Common Mode input voltage to the IA is close to zero.

[that is both inputs should be at the same potential, ensure that its also within the CMV range of the IA you are using]

Any bridge signal, which is in the order of mVolts will be superimposed on this CMV difference and will be amplified by the IA.

eg: if your CMV difference was say, 0.1v and your IA gain was 50, you would have a steady state output voltage of 5V and say if the bridge loading signal was only 1mV, you would have a 0.05v signal sitting on top of a 5v voltage level.
Also look at the datasheet for the IA's CMMR value.

Do you follow OK.?
Eric

 

[PRIYADHARSHINI PALANISAMY]

thanks ...how can we set V2..any constrain is there to set V2 according to our input voltage?

 

[crutschow]

Did you correct the op amp power polarities?